1. Field of the Invention
The present invention generally relates to a phase difference detecting apparatus for detecting a phase difference state obtained from a side beam of a three-beam system and, more particularly, to a phase difference detecting apparatus which can eliminate a phase difference error produced when a track that is swingable with respect to the direction perpendicular to the side beam is inverted.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings shows a structure of an optical head in which a laser beam is radiated onto recording pits on an optical disc from the optical head to thereby effect a tracking control and a focusing control. The optical head shown in FIG. 1 generally comprises one optical system block 10. The optical head can be moved along a radial direction (direction shown by an arrow A in FIG. 1) of an optical disc D on which there are formed spiral recording tracks. Within the optical block 10, a laser beam emitted from a laser diode 1 serving as a laser beam generating source is passed through a grating 2 and then introduced into a polarizing beam splitter 3. The laser beam passed through an analyzer surface 3a of the polarizing beam splitter 3 is introduced into a collimator lens 4, in which it is collimated to provide a collimated laser beam. Then, the collimated laser beam is passed through a quarter-wave plate 5 and introduced into an objective lens 6. Thus, the laser beam is converged on the optical disc D by the objective lens 6. The laser light beam introduced into the optical disc D is modulated and reflected on the recording tracks formed on the optical disc D and then converted into a reflected laser light beam.
The reflected laser light beam from the optical disc D is returned through the objective lens 6, collimated and then passed through the quarter-wave plate 5. The reflected laser light beam, which is reflected on the optical disc D and passed again through the quarter-wave plate 5, is rotated by .pi./2 in polarizing direction with respect to the laser light beam which is passed through the polarizing beam splitter 3 and focussed onto the optical disc D.
The reflected laser beam whose polarizing direction is rotated by .pi./2 is introduced into the collimator lens 4, in which it is converted into a converged laser light beam and then introduced into the polarizing beam splitter 3. Then, this laser light beam is reflected onto the analyzer surface 3a of the beam splitter 3 and then introduced through a sensor lens 7 to a photo-sensor unit 8. Then, the photo-sensor unit 8 detects the reflected laser beam modulated by the recording tracks formed on the optical disc D and outputs the change of the reflected laser beam as a signal.
The objective lens 6 includes a focusing control coil 9F which effects the focusing control by controlling the objective lens 6 to come closer or to come away from the optical disc D along the optical axis direction and a tracking control coil 9T which controls the objective lens 6 to move in the radial direction of the optical disc D, i.e., in the direction perpendicular to the optical axis of the objective lens 6.
In the above optical system block 10, the laser beam from the laser diode 1 is processed by the grating 2 to provide three laser light beams. Although the three laser light beams are shown as a single laser light beam in FIG. 1, in the light path succeeding to the grating 2 wherein the laser beam is reflected on the optical disc D, refracted by the polarizing beam splitter 3 and then introduced into the photo-sensor unit 8, there exist three laser beams in actual practice. The above three laser beams are introduced as a main beam which is used to read out an information which is recorded on the optical disc D at its recording track under the tracking control and two side beams located at both sides of the main beam so as to detect a tracking error of the main beam relative to the recording track.
The main beam and the two side beams are introduced into the optical disc D as follows. As shown in FIG. 2 of the accompanying drawings, under the proper tracking condition, a beam spot Bm of the main beam is formed such that its center becomes coincident with a center position of a recording track T. Also, beam spots Be and Bf of the two side beams are formed at symmetrical positions with respect to the direction extended along the recording track T and perpendicular to the recording track T about the beam spot Bm of the main beam so that they are partly overlapped on the recording tracks T.
The main beam and two side beams reflected on the optical disc D are supplied through cylindrical lenses forming a common sensor lens to a separated photo-detector provided in the photo-sensor unit 8.
As shown in FIG. 3 of the accompanying drawings, the photo-sensor unit 8 comprises a main beam photo-detector 11 composed of adjacent quadrant photo-detecting elements 11a, 11b, 11c and 11d and two side beam photo-detectors 12e and 12f spaced apart from these photo-detecting elements 11a, 11b, 11c and 11d. The main beam from the optical disc D is detected by the main beam photo-detector 11 and the two side beams from the optical disc D are detected by the side beam photodetectors 12e, 12f, respectively.
Detecting outputs Sa, Sb, Sc and Sd are generated from the photo-detecting elements 11a through 11d forming the main beam photo-detector 11 and then added by an adding unit 13. Therefore, the adding unit 13 derives a main beam detecting signal St corresponding to the main beam from the optical disc D. This main beam detecting signal St is used to form a focusing error signal. The side beam photo-detectors 12e and 12f derive side beam detecting signals Se and Sf corresponding to the respective two side beams. These side beam detecting signals Se and Sf are used to form a tracking error signal.
The main beam and two side beams introduced into the optical disc D are placed properly so that the beam spot location relationship shown in FIG. 2 is achieved under the proper tracking condition. Further, the main beam and two side beams reflected on the optical disc D have considerably different reflection intensity either when the reflected position is located on the pits on the optical disc D, i.e., the portion in which the recording track T is formed or when it is located between the recording tracks T.
When the recording track T is moved in the arrow L or R direction in FIG. 2 with respect to the main beam and two side beams, the main beam and two side beams respectively cross a plurality of recording tracks T relatively, whereby the main beam detecting signal St and the side beam detecting signals Se, Sf are respectively fluctuated in level at cycles corresponding to the spacing between the recording tracks T and the moving speed of the recording track T.
When the recording track T is moved in the arrow L direction with respect to the main beam and two side beams, the main beam detecting signal St presents a level fluctuation shown in FIG. 4B and the side beam detecting signal Se presents a level fluctuation which is advanced in phase by 90.degree. from the main beam detecting signal St as shown by a solid line in FIG. 4A. The side beam detecting signal Sf has a level fluctuation whose phase is delayed by 90.degree. from the main beam detecting signal St as shown in FIG. 4C.
When the recording track T is moved in the arrow R direction (FIG. 2) relative to the main beam and the two side beams, if the main beam detecting signal St has a level fluctuation (the same as that shown in FIG. 4B) shown in FIG. 5B, the side beam detecting signal Se has a level fluctuation whose phase is delayed by 90.degree. from the main beam detecting signal St as shown by a solid line in FIG. 5A. The side beam detecting signal Sf has a level fluctuation whose phase is advanced by 90.degree. from the main beam detecting signal St as shown by a solid line in FIG. 5C.
In the state such that the positions of the main beam and the two side beams are properly set relative to the optical disc D, when the recording track T is moved in the arrow L or R direction in FIG. 2 relative to the main beam and the two side beams, the side beam detecting signal Se and the side beam detecting signal Sf produce level fluctuations having a phase difference of 180.degree. therebetween.
The positions of the main beam and the two side beams relative to the optical disc D are set by adjusting a rotational angle of the grating 2, for example, under the condition such that the optical block 10 is attached to a predetermined position. If there is no error in attaching the optical system block 10 or error in adjusting the rotational angle of the grating 2, for example, the positional relation between the beam spot Bm of the main beam and the beam spots Be and Bf of the two side beams are set in proper states shown by solid lines in FIG. 6 under the condition such that the optical disc D is set in the proper tracking condition. However, as shown by one-dot chain lines in FIG. 6, it may be that due to error, the beam spots Be and Bf of the two side beams are displaced toward the outside of the recording track T as Be.sub.l and Bf.sub.1 from the proper positions. This state will hereinafter be referred to as "opened state". Alternatively, as shown by two-dot chain lines in FIG, 6, the positions of the beam spots Be and Bf of the two side beams may respectively, be erroneously displaced toward the inside of the recording track T as Be.sub.2 and Bf.sub.2 from the proper states. This state will hereinafter be referred to as "closed state" Consequently, it is frequently observed that the positions of the main beam and the two side beams relative to the optical disc D are not set correctly.
When the beam spot Bm formed by the main beam and the beam spots Be and Bf formed by the two side beams are set in "opened state", if the recording track T is moved in the arrow L direction in FIG. 2 relative to the main beam and the two side beams, then the side beam detecting signal Se has a level fluctuation whose phase is advanced as compared with the level fluctuation of the proper state as shown by a one-dot chain line in FIG. 4A. On the other hand, the side beam Sf has a level fluctuation whose phase is delayed as compared with the level fluctuation of the proper state as shown by a one-dot chain line in FIG. 4C.
If the recording track T is moved in the arrow R direction in FIG. 2 relative to the main beam and the two side beams, the side beam detecting signal Se has a level fluctuation whose phase is delayed as compared with the level fluctuation of the proper state as shown by a one-dot chain line in FIG. 5A. On the other hand, the side beam detecting signal Sf has a level fluctuation whose phase is advanced as compared with the level fluctuation of the proper state as shown by a one-dot chain line in FIG. 5C.
When the positional relationship between the beam spot Bm of the main beam and the beam spots Be, Bf of the two side beams is set in "closed state", if the recording track T is moved in the arrow L direction in FIG. 2 relative to the main beam and the two side beams, the side beam detecting signal Se has a level fluctuation whose phase is delayed as compared with the level fluctuation of the proper state as shown by a two-dot chain line in FIG. 4A. On the other hand, the side beam detecting signal Sf has a level fluctuation whose phase is advanced as compared with the level fluctuation of the proper state as shown by a two-dot chain line in FIG. 4C.
If the recording track T is moved in the arrow R direction in FIG. 2 relative to the main beam and the two side beams, then the side beam detecting signal Se has a level fluctuation whose phase is advanced as compared with the level fluctuation of the proper state as shown by a two-dot chain line in FIG. 5A. On the other hand, the side beam detecting signal Sf has a level fluctuation whose phase is delayed as compared with the level fluctuation of the proper state as shown by a two-dot chain line in FIG. 5C.
As described above, with respect to the optical system block 10, it is determined whether the positional relationship between the beam spot Bm of the main beam and the beam spots Be, Bf of the two side beams in the optical disc D is set in "proper state", "opened state" or "closed state". Further, if it is set in "opened state" or "closed state", the degree of the "opened state" or "closed state" must be detected. These are requirements in order to correct the position of the main beam and the two side beams relative to the optical disc d and to effect the setting for the tracking control.
During the above detection, the recording track T is moved in the direction shown by the arrow L or R in FIG. 2 due to eccentricity caused when the optical disc D is rotated under the condition such that the tracking control is not effected. Under this condition, the phase difference between the side beam detecting signal Se and the side beam detecting signal Sf is calculated.
However, the recording track T is swung relative to the main beam and the two side beams due to the eccentricity of the rotating optical disc D. Therefore, the swinging direction of the recording track T relative to the main beam and the two side beams is changed from the arrow L direction to the arrow R direction in FIG. 2 or vice versa at every half-rotation cycle of the optical disc D. Consequently, on the basis of the phase difference between the side beam detecting signal Se and the side beam detecting signal Sf, it can be determined whether the positional relationship between the beam spot Bm of the main beam and the beam spots Be and Bf of the two side beams is set in "proper state", "opened state" or "closed state". Also, the degree of the "opened state" or the "closed state" can be detected. However, it cannot be detected whether the side beam detecting signals Se and Sf are delayed or advanced in phase. Accordingly, it cannot be determined whether the positional relationship between the beam spot Bm of the main beam and the beam spots Be, Bf of the two side beams is set in the "opened state" or the "closed state".
The assignee of the present application has previously proposed a phase difference detecting apparatus which can adequately determine whether the positional relationship between the beam spot of the main beam and the beam spots of the two side beams is set in the "proper state", the "opened state" or the "closed state" and which can also detect the degree of the "opened state" or the "closed state" (see Japanese Laid-Open Patent Publication No. 2-56743).
FIG. 7 of the accompanying drawings is a block diagram showing a circuit arrangement of the previously-proposed phase difference detecting apparatus (Japanese Laid-Open Patent Publication No. 2-56743). In FIG. 7, the elements and parts within the photo-sensor unit 8 are marked with the same references in FIG. 3 and therefore need not be described in detail.
Under the proper tracking state, the main beam and the two side beams are introduced from the optical system block 10 into the optical disc D such that the beam spot Bm of the main beam and the beam spots Be, Bf of the two side beams are formed on the optical disc D as shown in FIG. 2.
As shown in FIG. 7, the main beam detecting signal St from the adding unit 13 in the photo-sensor unit 8, the side beam detecting signal Se from the side beam photo-detecting unit 12e, and the side beam detecting signal Sf from the side beam photo-detecting unit 12f are respectively supplied to comparing input terminals of level comparators 21, 22 and 23. Reference input terminals of these level comparators 21, 22 and 23 are respectively held at ground potentials. The level comparators derive waveform-shaped output signals Pt, Pe and Pf, respectively.
The waveform-shaped output signals Pt and Pe from the level comparators 21 and 22 are respectively supplied to a clock terminal C and a data terminal D of a D-type flip-flop (D-FF) 24. The D-type flip-flop 24 forms a phase comparing unit which determines whether the phase of the waveform-shaped output signal Pe is advanced or delayed from the phase of the waveform-shaped output signal Pt. A compared output signal Ss is output from an output terminal Q of the D-type flip-flop 24 and then supplied to a control terminal of a switching unit 25 which incorporates therein switches 25a, 25b, 25c and 25d, each of which is operated in a ganged fashion.
The waveform-shaped output signal Pe from the level comparator 22 is supplied through the switches 25a and 25b of the switching unit 25 to a data terminal D and a clock terminal C of a D-type flip-flop 26 and is also supplied to one input terminal of an exclusive-OR circuit (EX-OR) 27. The D-type flip-flop 26 forms a phase comparing unit which detects a mutual phase relationship between the waveform-shaped output signals Pe and Pf. A compared output signal Sp is output from an output terminal Q of the D-type flip-flop 26 and then delivered to an output terminal 28.
The waveform-shaped output signal Pf from the level comparator 23 is supplied to and inverted by an inverter 29 as a polarity inverted waveform-shaped output signal 29a. This output signal 29a is supplied through the switches 25c and 25d of the switching unit 25 to the clock terminal C and the data terminal D of the D-type flip-flop 26 and is also supplied to the other input terminal of the exclusive-OR circuit 27. A pulse output signal Pd is output from the exclusive-OR circuit 27 and supplied to a low-pass filter (LPF) 30 whose output signal Sh is delivered to an output terminal 31.
Due to the eccentricity in the spindle diameter and the central aperture diameter when the central aperture of the disc D is inserted into the spindle of a turntable and the disc D is rotated under the condition that the tracking is not effected, there are obtained the main beam detecting signal St and the side beam detecting signals Se and Sf. When the beam spots Bm and the side beam spots Be, Bf of the main and side beams are set in the "opened state" as the beam spots Be.sub.1 and Bf.sub.1 as shown by the one-dot chain line in FIG. 6, during a half-rotation period HL where the recording track T is moved in the arrow L direction in FIG. 2 relative to the main beam and the two side beams during each rotation period of the rotating optical disc D and during the next half-rotation period HR where the recording track T is moved in the arrow R direction in FIG. 2, the waveform-shaped output signal Pt obtained from the level comparator 21 based on the main beam detecting signal St becomes a square wave waveform-shaped output signal Pt whose waveform is shown in FIG. 8A.
The waveform-shaped output signal Pe obtained from the level comparator 22 based on the side beam detecting signal Se becomes a square wave signal whose phase is advanced by a phase angle larger than 90.degree. relative to the waveform-shaped output signal Pt during the half-rotation period HL and becomes a square wave signal whose phase is delayed by a phase angle larger than 90.degree. relative to the waveform-shaped output signal Pt during the half-rotation period as shown in FIG. 8B.
The polarity inverted waveform-shaped output signal 29a, which results from inverting the waveform-shaped output signal Pf obtained from the level comparator 23 based on the side beam detecting signal Sf by the inverter 29, becomes a square wave signal whose phase is advanced by a phase angle smaller than 90.degree. relative to the waveform-shaped output signal Pt during the half-rotation period HL and becomes a square wave signal whose phase is delayed by a phase angle smaller than 90.degree. relative to the waveform-shaped output signal Pt during the half rotation period HR as shown in FIG. 8D.
Consequently, the D-type flip-flop 24 supplied with the waveform-shaped output signals Pt and Pe detects the level of the waveform-shaped output signal Pe obtained at the leading edge of the waveform-shaped output signal Pt. The compared output signal Ss from the D-type flip-flop 24 is held at a positive constant value during the half-rotation period HL and held at a negative constant value during the half-rotation period HR as shown in FIG. 8C.
In the switching unit 25 whose control terminal is supplied with the compared output signal Ss from the D-type flip-flop 24, the switches 25a, 25c are turned on and the switches 25b, 25d are turned off during the half-rotation period HL where the compared output signal Ss is held at a positive constant value. Also, the switches 25b, 25d are turned on and the switches 25a, 25c are turned off during the half-rotation period HR where the compared output signal Ss is held at a negative constant value.
During the half-rotation period HR, the waveform-shaped output signal Pe is supplied through the switch 25b in the switching unit 25 to the clock terminal C of the D-type flip-flop 26. Also, the polarity inverted waveform-shaped output signal 29a is supplied through the switch 25d in the switching unit 25 to the data terminal D of the D-type flip-flop 26. Therefore, the D-type flip-flop 26 detects the level of the polarity inverted waveform-shaped output signal 29a to thereby detect whether the phase of the polarity inverted waveform-shaped output signal 29a is advanced or delayed relative to the phase of the waveform-shaped output signal Pe. Consequently, the compared output signal Sp from the D-type flip-flop 26 is held at a positive constant value as shown in FIG. 8E.
As described above, the switching unit 25 forms the phase comparison control unit which controls the phase comparing operation mode in the D-type flip-flop 26 in response to the polarity of the compared output signal Ss obtained from the D-type flip-flop 24. Therefore, the compared output Sp obtained from the D-type flip-flop 26 is held at the positive constant value both in the half-rotation periods HL and HR.
The pulse output signal from the exclusive-OR circuit 27 which is supplied with the waveform-shaped output signal Pe and the polarity inverted waveform-shaped output signal 29a is converted into a pulse train having a width corresponding to a period where one of the waveform-shaped output signal Pe and the polarity inverted waveform-shaped output signal 29a is held at high level and the other is held at low level, i.e., a width corresponding to the phase difference between the waveform-shaped output signal Pe and the polarity inverted waveform-shaped output signal 29a both in the half-rotation periods HL and RL as shown in FIG. 8F. Thus, as shown in FIG. 8G, the output signal Sh from the low-pass filter 30 has a level corresponding to the phase difference between the waveform-shaped output signal Pe and the polarity inverted waveform-shaped output signal 29a.
Similarly, in the "closed state", the waveforms whose phases are different as shown in FIGS. 9A through 9G are presented similarly to FIGS. 8A through 8G. In the "closed state", the compared output signal Sp appears as a negative constant value.
That is, the phase-advanced or phase-delayed condition of the beam spots Be, Bf of the two side beams is supplied to the output terminal 28 as the phase-advanced and phase-delayed signal Sp. In the "opened state", the signal Sp is held at a positive value and in the "closed state", the signal Sp is held at a negative value.
Further, since the output signal Sh from the low-pass filter 30 becomes an absolute value detecting signal representative of a phase difference between the side beam detecting signal Se and the side beam detecting signal Sf, the absolute value of the phase can be obtained.
According to the above-mentioned conventional arrangement, the phase advance and delay discriminating signal for discriminating the advanced phase or delayed phase between the side beam detecting signals or both the advanced phase or delayed phase and a phase difference of the main beam detecting signal and the side beam detecting signals formed with respect to two side beams obtained from the reproducing system of the three-beam system optical disc through the photo-sensor unit 8 can be obtained by a relatively simplified arrangement. However, there is then the problem such that, when an eccentricity is produced due to a clearance between the spindle and a central aperture diameter of the optical disc D, a measurement accuracy becomes deteriorated to such an extent that it cannot be made negligible due to the phase difference of eccentricity in the vicinity of the portion where the phase difference between the waveform-shaped output signals Pe and Pf from the photo-sensor unit 8 is substantially zero.
When a maximum value of a phase difference generated by the eccentricity of the optical disc D is selected to be .DELTA..alpha..sub.max and a phase difference between the signals Pe and Pf is selected to be .alpha.D.degree. , if .alpha.D.gtoreq..DELTA..alpha..sub.max is satisfied, then a phase change Vd.sub.1 caused by the eccentricity does not exceeds 0.degree. as shown in a Lissajous waveform diagram in FIG. 10A and changes with a constant polarity so that substantially accurate value can be obtained. Therefore, there occurs no problem if it is determined whether the compared signal output is positive or negative and if the value of the phase absolute value Sh is measured, respectively.
If .alpha.D&lt;.DELTA..alpha..sub.max is satisfied, a phase difference change Vd.sub.2 due to the eccentricity exceeds 0.degree. and is changed as shown in FIG. 10B. As a result, it cannot be accurately determined whether the compared signal output Sp is positive or negative. Thus, an error occurs in the measured phase difference.
If a difference between measured data used to calculate the phase difference absolute value and to determine whether the compared signal output Sp is positive or negative and a true value is calculated in simulation fashion, we have the following results shown in FIGS. 11A, 11B.
FIG. 11A shows an eccentricity waveform obtained when the optical disc D is rotated once under the condition that .alpha.D&lt;.DELTA..alpha..sub.max satisfied. In this case, the real measured values and the true values of the phase differences represent substantially equal values.
FIG. 11B shows an eccentricity waveform obtained when the optical disc D is rotated once under the condition that .alpha.D&lt;.DELTA..alpha..sub.max is satisfied. As shown in FIG. 11B, an absolute value of the phase difference becomes a value which results from averaging the portions shown hatched. Therefore, the polarity of the compared signal output Sp cannot be determined accurately. By way of example, when the eccentricity amount of the optical disc D is taken as .DELTA.y and a difference between the true value .alpha.D and the measured value .alpha. is calculated, if a radius of the optical disc D is r, a phase difference between the side beam detecting signals Se, Sf of the photo-sensor unit 8 is taken as .DELTA..alpha. and one sampling is taken as n, then the true value .alpha.D, the measured value .alpha. and a difference .alpha.-.alpha.D between the true value .alpha.D and the measured value .alpha. are illustrated on the following table where the sampling value n=36, the radius r=24 and the eccentricity value .DELTA.y=30 .mu.m.
TABLE ______________________________________ .alpha.D 0.degree. 2.degree. 5.degree. 7.degree. 10.degree. 12.94.degree. .alpha. 8.22.degree. 8.33.degree. 9.54.degree. 9.55.degree. 10.82.degree. 12.94.degree. .alpha.-.alpha.D 8.22 6.33 4.54 2.55 0.82 0 ______________________________________
Having compared a relationship between the true value .alpha.D and the measured value .alpha. with an ideal line, it is to be understood that an error occurs in the vicinity of the phase difference 0.degree. as shown in FIG. 12.